The present invention is directed to an encoder, and more particularly, to an encoder having a spacer to maintain a predetermined distance between the encoder mask and the encoder film.
Optical encoders are used to transform the motion of various components into an electronic waveform to track the location and velocity of the components. For example, printers and other business machines may include encoders to track, for example, the rotational position and velocity of a feedroll shaft, or the linear position and velocity of a printhead. Such encoders typically include a light source (such as a light emitting diode (xe2x80x9cLEDxe2x80x9d)) and a photosensor or photodetector (such as a photodiode) to detect light emitted by the light source. The encoder further typically includes an encoder film (such as an encoder strip or encoder wheel) located between the light source and the photodetector, and a mask located between the light source and the photodetector. The encoder film is typically coupled to the shaft, printhead or other component to be tracked such that the encoder film rotates with the shaft or moves with the printhead. The encoder and the mask each usually include a series of alternating bars and windows printed thereon.
In many existing encoders, the mask is embedded in the photodetector. For example, the mask may be etched over the detecting element of the photodetector. However, when the mask is located internally to the photodetector, the encoder typically must include optics to process (i.e. xe2x80x9ccolumnizexe2x80x9d) the light entering the photodetector. Because optics can be expensive, it may be desired to move the mask externally of the photodetector and adjacent to the encoder strip, which eliminates the need for optics to process and columnize the light.
However, when the mask is moved externally of the photodetector, a tight tolerance must be maintained between the mask and the encoder film to ensure high overall accuracy for the encoder. For example, it is desirable to maintain the mask and encoder film as close as possible to each other without contact in order to ensure that the light passing through the mask and film is colonized. However, the mask and encoder film should remain spaced apart to avoid rubbing off the bars printed on the mask or encoder film, and to avoid other undesired effects that may be caused by friction between the mask and encoder film.
The present invention is an encoder which includes an encoder film, a mask, and a spacer located between the mask and the encoder film to maintain a set spacing between the mask and the encoder film. In one embodiment the invention is an encoder including a light source for emitting light, a photodetector for detecting at least part of the light emitted by the light source, and an encoder film including a plurality of alternating bars and windows and being located between the light source and the photodetector. The encoder includes a mask including a plurality of alternating bars and windows, and the mask is located between the light source and the photodetector. The encoder further includes a spacer located between the mask and the encoder film, the encoder film being biased against the spacer to maintain a predetermined spacing between the mask and the encoder film.